This invention relates to a method and a device for the measurement of exhaust gas from internal combustion engines (gasoline engines and diesel engines) according to a full-flow dilution method. The full flow dilution method is often called CVS method (Constant Volume Sampling method) because the current regulations require a CVS method which is a full flow dilution method with constant volume sampling. The method and device of the invention are suitable both for a gravimetric measurement of particulates in the exhaust gas, as well as, for a measurement of gaseous exhaust components in the emissions.
Laws and regulations in Europe and in the United States of America limit the emission of gaseous and solid/liquid pollutant components in the exhaust of gasoline and diesel engines. According to current regulations, the exhaust gas from an engine must be diluted in a CVS (Constant Volume Sampling) system.
The particulate components emitted by diesel engines are measured by a gravimetric total content measurement over the test period, i.e. a special particulates measurement filter is weighed before and after it has been charged with diluted exhaust gas from the diesel engine. The specific particulates emissions are calculated from the weight difference, taking into consideration the length of the test line (driving distance) and the work performed.
The gaseous components of the exhaust from gasoline engines are also measured in the diluted exhaust using a CVS system. In this case, the diluted exhaust gas is passed into a collecting bag, and after a test cycle has been completed, the concentration of the individual gaseous components of the exhaust in the collecting bag is determined.
In recent years, the limits for both particulates and gaseous pollutants have been drastically reduced. Consequently, the difference between the absolute amount of the measured quantity and the measurement resolution is constantly being reduced, as a result of which the accuracy of the measurement is likewise decreasing.
With regard to the applicable European provisions and the related specific requirements for measurement and instrumentation, reference is made to Directive 98/69/EC of the European Parliament and of the Council of 13 Oct. 1998 relating to measures to be taken against air pollution by emissions from motor vehicles and amending Council Directive 70/220/EEC; Directive 1999/96/EEC of the European Parliament and of the Council of 13 Dec. 1999 on the approximation of the laws of the Member States relating to measures to be taken against the emission of gaseous and particulate pollutants from compression ignition engines for use in vehicles, and the emission of gaseous pollutants from positive ignition engines fuelled with natural gas or liquefied petroleum gas for use in vehicles and amending Council Directive 88/77/EEC; Council Directive 91/441/EEC amending Directive 70/220/EEC on the approximation of the laws of the Member States relating to measures to be taken against air pollution by emissions from motor vehicles, 26 Jun. 1991; Commission Directive 2003/76/EC of 11 Aug. 2003 amending Council Directive 70/220/EEC relating to measures to be taken against air pollution by emissions from motor vehicles. Corresponding US regulations are to be found in: Code of Federal Regulations, Title 40, Part 86, Subpart B—Emission Regulations for 1977 and Later Model Year New Light-Duty Vehicles and New Light-Duty Trucks; 1999.
In a full-flow dilution system using the prescribed Constant Volume Sampling CVS, in general the total exhaust gas volume flow from a test engine is mixed with dilution air in a dilution tunnel so that the total volume flow, which is composed of the sum of the exhaust gas volume flow and the dilution air volume flow, remains constant. The dilution of the exhaust must be high enough so as to prevent the condensation of water from the exhaust both in the dilution tunnel and during the sampling, whereby a maximum temperature at the particle filter may not be exceeded during the particle emission measurement.
The applicable CVS condition in the system is as follows:
Exhaust volume flow x dilution ratio=constant
or
Exhaust volume flow+dilution air volume flow=constant
The dilution ratio DR is defined as:
DR=(dilution air flow+exhaust volume flow)/exhaust volume flow
The CVS condition results in a high dilution ratio for operating points with a low exhaust volume flow and a low dilution ratio for operating points with a high exhaust volume flow.
The advantage of the CVS principle consists primarily in the fact that, although the volume flow of the exhaust from an internal combustion engine is variable, the total volume flow consisting of the exhaust and air is held constant—thereby allowing the determination of the particulate emissions or the concentration of gaseous pollutants in a simple manner. There is no need to go into the details of the determination, which is well known to a technician skilled in the art and also described in the above referenced EU Directives and US Regulations.
A conventional CVS system as illustrated in FIG. 1 for a diesel engine 2, for example, consists of the following individual components: dilution air filter 3, dilution tunnel 1, sampling unit and suction or intake unit in form of a blower 6. The mixing of ambient air and exhaust takes place in the dilution tunnel 1, where a mixing orifice 4 is commonly used to promote a thorough mixing. At the end of the tunnel is the suction unit, which consists either of a positive displacement pump or the blower 6, which delivers a constant volume flow, or which consists of a blower, for example, which can optionally be downstream from one or more venturis. Independently of the blower, a heat exchanger 5 is located upstream from the suction unit to ensure the constancy of the volume flow and to protect the blower or pump, respectively.
The sampling unit is used to collect the particulates on a filter 7. For that purpose, a defined partial sampling flow of the diluted exhaust is extracted or drawn from the dilution tunnel 1 by means of a sampling probe with the assistance of the sampling pump 9. During a particulate emissions measurement, the partial sampling flow is passed through a filter holder containing primary and secondary particulate filters 7, and is recorded by means of a suitable instrument for the measurement of volume and mass, respectively. The partial sampling flow is drawn by a pump through a volume measuring device 8 that is located upstream from the filter. For the measurement of the gaseous components, which is not illustrated here, the partial sampling flow is pumped into bags.
Various solutions have been proposed in terms of measurement equipment for the purpose of measuring the lower emissions levels of engines. However, quite a few of the solutions are based on systems that use a dilution of a small fraction of the total flow, such as the methods disclosed in DE 41 21 928 A1 and U.S. Pat. No. 6,112,575, for example. In these methods of the prior art, the exhaust volume flow is precisely measured continuously, and an attempt is made to adjust the dilution of the partial sampling flow so as to achieve a better measurement accuracy. In the Bag Mini Diluter (BMD) promoted by the United States Environmental Protection Agency (EPA), the current exhaust volume flow must be known at every point during the test cycle. In this system, the dilution ratio is kept constant at every point during the test cycle, so that the diluted fraction or portion of the exhaust volume flow must be adjusted sufficiently dynamically to guarantee the constancy required for accurate measurement results.
Periodically measuring the exhaust volume flow, for example, for every second during the test with such partial sampling or partial flow dilution systems is problematic, because the exhaust volume flow generally can only imprecisely be measured, whereas such measurement is absolutely essential for an evaluation of the pollutant emissions in the systems described in the preceding section. These drawbacks also hold true for the partial flow dilution system of WO 9601998 A in which an exactly constant dilution ration is provided by proportional control of a non-diluted partial exhaust gas flow and a dilution gas both being regulated by pressure control means. The exhaust gas is measured per second in a measuring tube. For allowing extraction of the non-diluted partial exhaust flow in the measuring tube, an element for achieving a calm flow is provided which element eliminates pressure pulsations. Otherwise, the desired small exhaust gas partial amounts of, for example, only 1 percent of the undiluted exhaust volume flow could not be reliably extracted.
One advantage of the CVS systems, on the other hand, is that it is not necessary to determine the exhaust volume flow during the test. As described above, in the CVS systems the dilution ratio in the dilution tunnel changes constantly, because the exhaust volume flow varies continuously during a test cycle, although the total volume flow remains constant. Because the dilution ratio must be adjusted for the highest exhaust volume flow that occurs during the test cycle such that condensation of water is prevented, the dilution ratios are principally high for lower exhaust volume flows. Furthermore, because a constant partial sampling flow of the diluted exhaust is extracted, there are very low pollutant concentrations available for the measurement performed using the particulates filter or the collecting bag.
U.S. Pat. No. 6,405,577 describes a full-flow dilution CVS system in which this problem is discussed in regard to the determination of gaseous pollutant concentrations. Variable venturis are used to vary both the total volume flow as well as the partial sampling flow diverted from the total flow, as a function of the operating conditions of the vehicle being tested. For that purpose, a control unit is used that takes into consideration the different operating conditions during the test cycle, as well as, optionally, information on the current operating conditions supplied directly from the vehicle control system. The operating conditions are thereby divided into phases, whereby in each phase a suitable constant total volume flow is set by means of the variable venturi. The partial volume flow is correspondingly adjusted so that a constant ratio between partial flow and total volume flow results.
The total volume flow is determined by means of a detector mechanism combined with the variable venturi. The current flow rate of the total volume flow is calculated on the basis of the pressure and the temperature at the input of the total volume flow venturi, and with reference to the current setting of the variable venturi. For this purpose, there are provided pressure and temperature sensors in the above-mentioned detector mechanism.
In addition, in order to accurately determine the concentrations of gaseous pollutants, the flow rate of the exhaust is calculated by subtracting the current flow rate of the fresh air fed into the system from the instantaneous flow rate in the total volume flow. The venturi is controlled as a function of the specified operating conditions in several pre-defined stages; and the total volume flow produced is continuously calculated from the venturi settings and detected temperature and pressure values, and is used to adjust the partial sampling flow.
The stages or levels in which the total volume flow is kept at a constant value according to the CVS method are adjusted so that the exhaust volume flow does not exceed the total volume flow at any point. As a rule, therefore, the dilution ratio in the individual stages is not identical. In other words, in this system a test cycle is divided into sub-cycles of different volume flow levels each fulfilling CVS requirements, whereby the sub-cycles are adjusted as a function of the operating conditions.